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Abstract:

Taught is a high sensitivity THz detector and camera comprising an
integration body of a photonic crystal THz micro-cavity and semiconductor
transistor on a semiconductor base. The THz signal is localized inside of
the photonic crystal micro-cavity so as to generate high intensity field
in the micro-cavity. The heat effect of the THz wave therein produces
electron-hole pairs in the semiconductor. The current carrier is injected
into the base electrode of the transistor and is amplified therein to
produce signal current in the external circuit and thus high sensitive
THz signal detection is realized. Integrating many such detectors
together to construct a THz resonant cavity array, each resonant cavity
only receives THz light from a certain position and having certain
intensity. The signal is converted and then stored to obtain a complete
THz image so as to realize imaging in real-time. Special signal
amplification circuit is used to eliminate THz background radiation
noise.

Claims:

1. A THz signal detector or camera comprising a photonic crystal and a
plurality of THz micro-cavities.

2. The THz signal detector or camera of claim 1, wherein said photonic
crystal THz micro-cavity receives, localizes, and amplifies THz signal,
thereby generating high intensity field in said micro-cavity.

3. The THz signal detector or camera of claim 1, wherein said photonic
crystal and said plurality of THz micro-cavities are made of
semiconductor poles or metal poles.

5. The THz signal detector or camera of claim 4, wherein current carriers
are injected into a base electrode of the transistor so as to generate
signal current in an external circuit to detect the THz wave signal.

6. The THz signal detector or camera of claim 1, wherein each micro-cavity
is equivalent to a detector and receives a THz wave from a certain
position with a certain intensity.

7. The THz signal detector or camera of claim 1, wherein a THz signal is
localized in the cavity and is amplified proportionally, so that the
sensitivity to THz light is improved.

8. The THz signal detector or camera of claim 5, wherein the same
quantities of semiconductor units and transistor units are allocated
below the photonic crystal THz micro-cavity area array.

9. The THz signal detector or camera of claim 8, wherein under the
stimulation of THz wave, the semiconductor units will release
electron-hole pairs and thus generate signal current.

10. The THz signal detector or camera of claim 9, wherein the signal
current is connected to a signal acquisition circuit where electronic
switches are used and the acquired signal is then input into an A/D
converter.

11. The THz signal detector or camera of claim 10, wherein the THz signal
is finally stored into a RAM to obtain a complete THz image.

12. The THz signal detector or camera of claim 1, wherein said
semiconductor unit is a Si material, a GaAs material, or other
semiconductor material.

13. The THz signal detector or camera of claim 1, wherein the current
carrier generated by the heat effect of THz wave in the semiconductor
unit is injected firstly into the base electrode of a transistor or the
PN junction of a diode, and then is connected to an external circuit.

14. The THz signal detector or camera of claim 1, whereinsaid high
refractive index medium in the photonic crystal is Si or GaAs, or other
semiconductor material, or metal material the low refractive index medium
is air;the base is ceramic or sapphire;the micro-cavity is obtained by
changing the refractive index of medium or the radius of the medium rod;
andthe conductor layout or connection configuration of the electric
conducting wires connecting the transistors or diodes for supplying the
bias voltage is of various types.

15. The THz signal detector or camera of claim 1, wherein by changing the
duty ratio of high and low refractive index mediums, the structure and
lattice constant of photonic crystal, the defect mode in the micro-cavity
is adjusted to any frequency in THz range according to demand.

16. The THz signal detector or camera of claim 1, wherein said
micro-cavity is a two-dimensional photonic crystal micro-cavity based on
a tetragonal lattice, a trigonal lattice, a honeycomb structure lattice,
a quasi-crystal, or other any structure.

17. The THz signal detector or camera of claim 1, wherein the lattice
constant of the photonic crystal and the wavelength of the THz light are
in the same order of magnitude, and the photonic crystal micro-cavity is
manufactured with etching technology or other mechanical holing method.

18. The THz signal detector or camera of claim 2, wherein a P section and
an N section is made on the semiconductor unit by means of diffusion to
form an NPN transistor or a PNP transistor.

19. The THz signal detector or camera of claim 5, wherein a P section and
an N section is made on the semiconductor unit by means of diffusion to
form an NP junction or a PN junction.

20. The THz signal detector or camera of claim 1, wherein the background
THz noise signal is eliminated by adopting special circuit connection
method and adjusting the bias voltage.

[0003]The invention relates to signal detectors and cameras having high
sensitivity in the THz range, and more particularly, to high sensitivity
THz signal detectors and cameras employing photonic crystal technology.

[0004]2. Description of the Related Art

[0005]The term "THz wave" refers to electromagnetic radiation with
frequency in the range of 100 GHz-10 THz (which corresponds to a
wavelength of 30 μm-3 mm). This frequency is located between microwave
frequency and infrared frequency. Due to the absence of effective THz
wave generation and detection methods, the knowledge about the
performance of THz band electromagnetic radiation is limited. This gave
rise to a "THz gap" in the electromagnetic spectrum.

[0006]With the rapid development of ultra-fast laser technology over the
past several years, a stable and reliable way for generating THz pulse is
provided. Thus, the study on THz wave radiation mechanism is promoted and
the relevant detection and application technology flourish.

[0007]Compared with conventional light sources, the THz pulse light source
has the following properties:

[0008]a) the THz wave has a pulse width of about 0.1-10 ps magnitude,
which can be used not only for transient spectrum study with a
subpicosecond time resolution, but also to avoid effectively the
disturbance of background radiation noise with the assistance of sampling
gate measurement technology. Currently, the signal to noise ratio for
measuring THz wave radiation strength is greater than 104 level;

[0009]b) the THz pulse source usually comprises electromagnetic
oscillation having more than one period; the bandwidth of a single pulse
can cover the range of GHz to scores of THz;

[0010]c) the THz wave having a very high time and space coherence can be
used as a camera light source, so that the amplitude and phase can be
obtained at the same time; and

[0011]d) the THz wave has a photon energy only in the millielectronvolt
range, so that the THz wave will not destroy a tested object, and thus
can be used for non-destructive testing.

[0013]The THz wave scanning delay system in conventional THz spectrum and
imaging technology mainly adopts point-by-point mechanical scanning or
electric translation scanning to realize time-delayed time domain
spectrum measurement and imaging with the help of computer composition.
This method has drawbacks such that an upper limit for scanning exists,
the scanning speed is insufficient, the spectrum information cannot be
observed timely, the time-space resolution is poor, and thus a real-time
imaging is difficult to realize. Besides, the hot background noise,
resulting from the slow scanning speed, cannot be suppressed effectively,
which limits the practical application of the THz imaging technology. For
example, for inspection at customs or safety checks at other confidential
places, the image signal needs to be obtained immediately, leading to the
requirement for developing practical THz detector and camera.

[0014]On the other hand, most current THz detection and imaging
technologies are realized by means of electro-optical modulating cell,
which also obstructs the integration of THz detector and camera.

[0015]Chinese Pat. Appl. No. CN 03116029.8 entitled "THz wave
two-dimensional electro-optic area array imaging method" introduces a
two-dimensional area array electro-optic crystal THz signal detector.
Though the time-space resolution and signal-noise ratio are good and
instant imaging can be realized, the detection sensitivity is not
satisfactory and the capability to detect low power THz signal is poor.

SUMMARY OF THE INVENTION

[0016]Therefore, it is one objective of this invention to provide a high
sensitivity THz signal detector and camera employing photonic crystal
micro-cavities, and methods for manufacturing and using thereof.

[0017]The method for manufacturing a high sensitivity THz signal detector
and camera comprises: (a) preparing a photonic crystal having a lattice
constant in the range of THz wavelength magnitude by using a
semiconductor or metal material; and (b) changing the dimension of one of
the dielectric or metal rods to form a defect to form a photonic crystal
THz micro-cavity. A frequency in the THz range can be obtained according
to demand by adjusting the lattice constant and the size of the defect.

[0018]The high sensitivity THz signal detector receives and localizes THz
signal via a single photonic crystal THz micro-cavity, which also
syntonizes and amplifies the THz signal. The semiconductor unit below the
micro-cavity functions to generate electron-hole pairs by a thermo-effect
due to the high intensity field inside of the micro-cavity. A signal
current is generated in the external circuit by injecting current
carriers into the base electrode of the transistor, and is proportional
to the THz wave intensity so that the THz wave signal can be detected.

[0019]For rapid photographing and real-time viewing, many such photonic
crystal THz micro-cavities are integrated into an area to from an array
of THz resonant cavities, being equivalent to a high sensitivity THz
signal detector area array. Irradiated by THz wave, each resonant cavity
receives a THz light from a specific position and having a specific
intensity, a THz signal relevant to the THz radiation intensity of the
specific position exists inside of each cavity, and the signal is
localized in the cavity and is amplified proportionally, so that the
sensitivity to THz light is greatly improved.

[0020]Allocating the same quantities of Si (optionally, GaAs or other
semiconductor) units and transistors (or diodes) below the photonic
crystal THz micro-cavity area array, these units will release
electron-hole pairs under the function of THz wave and thus generate
signal currents. The signal currents are collected by a signal
acquisition circuit where electronic switches are used; and the acquired
signal is then inputted into an A/D converter. The signal is finally
stored in a random access memory (RAM) to obtain a complete THz image.
The time used for acquiring a THz image is the same as that used for
imaging by a normal digital camera, so that imaging with a higher speed
can be obtained.

[0021]Another problem for imaging with THz wave is that much of the THz
background radiation noise in a natural environment with a normal
temperature is difficult to eliminate. A conventional way to overcome the
influence of the background noise is to apply the detector in a super-low
temperature environment, resulting in a large size of the detection
device and thus relatively high cost. A special signal extraction circuit
is adopted in the embodiments of this invention; the noise signal caused
by the THz background radiation can be eliminated by adjusting the bias
voltage of the regulating circuit.

[0022]As a result, the high sensitivity THz signal detector and camera of
this invention provides many advantages. The two dimensional THz wave
signal can be detected directly in an instant and thus the two
dimensional THz wave imaging occurs in real time. The major material used
for manufacturing of the units is Si semiconductor material so that it
can be integrated with other electronic components directly. In addition,
this invention provides very high sensitivity, good time and space
resolution, high signal-noise ratio, and good capability for integration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a side view of a high sensitivity THz signal detector or a
unit of a THz camera system according to one embodiment of this
invention;

[0024]FIG. 2 is a top view of a high sensitivity THz signal detector or a
unit of a THz camera system according to one embodiment of this
invention; and

[0025]FIG. 3 is a partial view of key parts of a high sensitivity THz wave
imaging camera according to one embodiment of this invention.

[0026]The reference numbers of the various parts shown in the drawings are
listed below, in which semiconductor Si or GaAs material corresponds to
the number 1 (the semiconductor poles and the defect poles in the
photonic crystal can also be replaced by metal poles); air--2; photonic
crystal defect area--3; and base ceramic, sapphire, or other proper
material--4.

[0028]As shown in FIG. 1, 1 is a high refractive index medium Si, 2 is low
refractive index medium air. The semiconductor Si is produced on a
ceramic base 4 using molecular beam epitaxy technology. The photonic
crystal is produced on the Si base using etching technology. (If the
photonic crystal is to be made of metal poles, a layer of metal is
further deposited on the Si base, and then metal poles are produced by
ion etching.) The lattice constant of the photonic crystal and the
wavelength of the THz light are in the same order of magnitude.
Optionally, the lattice constant of the photonic crystal can be selected
as 10 μm. The band gap of photonic crystal can be adjusted in THz
wavelength range by changing the duty ratio of medium 1. The defect 3,
and thus the photonic crystal THz micro-cavity, can be formed by changing
the size of one high refractive index medium.

[0029]FIG. 2 illustrates a top view of a photonic crystal micro-cavity.
The defect mode, of which the wavelength is chosen to be equal to that of
the THz signal, in the micro-cavity can be tuned to any position in the
band gap by changing the radius of the defect area 3. Therefore, the
structure parameters of the photonic crystal micro-cavity can be selected
according to the THz wavelength. NPN (or PNP) transistor is formed on the
Si base by the diffusion method. The transistor is connected with the
external circuit. Voltage is applied to the collector electrode and the
base electrode, the emitter electrode is connected to a differential
amplifier. The purpose of applying bias voltage Eb on the base
electrode is to increase the amplification coefficient of the transistor
and to have a good linear amplification characteristic. This applied
voltage is optional if the requirement is not high. The output signal is
connected to the receiver of an external circuit. The differential
amplifier can eliminate the current signal produced by the background
radiation so that the detector can be used for detecting the THz wave in
a room temperature environment.

[0030]B. The basic operation principle of the high sensitivity THz signal
detector.

[0031]Under THz irradiation, a localization of the THz wave is generated
in the defect area in the photonic crystal micro-cavity, and thus the THz
wave oscillates inside of the micro-cavity. The field intensity is high
enough to generate heat energy so that electron-hole pairs are produced
on the semiconductor Si base. Under the function of bias voltage, the
current carrier is injected into the base electrode of the transistor,
and is amplified to an observable current at the emitter electrode.
Through the differential amplifier, the current is connected to the
direct current amplifier of the external current and is amplified thereby
again, and is detected finally. Since the current detected is
proportional to the intensity of the incident wave, the THz wave signal
can be detected. The localization effect of the photonic crystal
micro-cavity can provide good detection sensitivity, and the transistor
can contribute to amplify the signal so that the detection sensitivity is
further improved. Therefore, this device functions as a high sensitivity
THz signal detector.

[0032]Applying many such point detectors to construct an array allows for
preparing the key part of a THz rapid imaging camera, as shown in FIG. 3.
Manufacturing a defect at an area of overlap of three high refractive
index dielectric poles in the photonic crystal realizes a two-dimensional
integrated THz micro-cavity array, namely, a high sensitivity THz signal
detector area array. The distance between each two micro-cavities is 40
μm (which can be adjusted for special operating wavelength). The
number of pixels of the reference CCD is 1024×1024, so that in the
design of m×n two-dimensional photonic crystal micro-cavity array,
m=1024 and n=1024, the size of the whole m×n two-dimensional
photonic crystal is about 4 cm×3.5 cm. Generally, the line and
arrow numbers of the two-dimensional photonic crystal micro-cavities
should be equal to the corresponding line and arrow numbers of pixels of
the CCD.

[0033]The base electrodes and collector electrodes of all transistors
share the same voltage source. The emitter electrode is connected with a
resistor and the differential amplifier, and the output terminal is
connected with the external circuit.

[0034]In real applications, in order to eliminate the interference of
electromagnetic waves from other wave bands, a filter is added on the
incident surface of the wave. This filter can prevent all other waves
from passing through except a THz wave.

[0035]The basic operation principle of high sensitivity THz signal camera
is described below. Because each cavity in the photonic crystal THz
micro-cavity array is equivalent to a detector and receives a THz wave
from a certain position, a THz signal relevant to the THz radiation
intensity of the specific position exists inside of each cavity, and the
signal is localized in the cavity and is amplified proportionally, which
improves the detection sensitivity to THz light greatly. The signal
current detected by each detector is outputted from different ports
respectively (shown as S1 to S6 in FIG. 3). All the signal
currents are connected to a signal acquisition circuit where electronic
switches are used; the acquired signal is inputted into an A/D converter,
and is then stored in a RAM, and is finally displayed on an electronic
screen. In this way, a complete THz image is obtained.

[0036]Although this invention has been described in connection with
preferred embodiments thereof, it will be appreciated by those skilled in
the art that additions, modifications, substitutions and deletions not
specifically described may be made without departing from the spirit and
scope of the invention as defined in the claims.